Atomic absorption spectroscopy system having sample dissociation energy control



, Jan. 27, 1970 J. F. RENDINA 3,

ATOMIC ABSORPTION SPECTROSCOPY SYSTEM HAVING SAMPLE. DISSOCIATION ENERGY CONTROL Filed Nov. 24, 1967 Atomic flbsorpliort,

Speqzrom Attorneus United States Patent O 3,492,074 ATOMIC ABSORPTION SPECTROSCOPY SYSTEM HAVING SAMPLE DISSOCIATION ENERGY CONTROL John F. Rendina, Kennett Square, Pa., assignor to Hewlett-Packard Company, Palo Alto, Calif., a

corporation of California Filed Nov. 24, 1967, Ser. No. 685,455 Int. Cl. G01j 3/30, 3/42 U.S. Cl. 35685 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method and apparatus for controlling the effective temperature of a plasma and, more particularly, to a method and apparatus for controlling the ability of a plasma to transfer energy.

BACKGROUND OF THE INVENTION Since the time of Fraunhoffer, it has been known that if a substance is converted into an atomic vapor it will absorb radiation of the same wave lengths as are emitted by the same substance when electrically or thermally excited. It is known also that if a given substance is heated sufficiently its vapor will emit certain predetermined characteristic radiation. These principles have been used to great advantage in the development of what is now known as atomic absorption spectroscopy. Usually, in atomic absorption spectroscopy, radiation from a characteristic line or other conventional source is passed through an atomic vapor of the sample under test. The extent or degree of absorption of the characteristic radiation by the atmoic vapor is a measure or indication of the presence of atoms having the same characteristic lines as the source. This type of spectroscopy is now used to measure and detect the presence of various metals in substances although the technique in theory is not limited to metals along. Even more recently these techniques have been extended to detect the presence of many organic compounds using metallic tracers.

To detect the presence of a particular metal or element in a sample, it is necessary to have a source of radiation which produces wave lengths of light which are characteristic of the element atoms under test. This source of radiation is usually produced by a source capable of providing narrow lines. One such source is the well known hollow cathode lamp. Another such source is a metallic halide discharge tube of the type described in U.S. Patent 3,319,119 isssued May 9, 1967 to John F. Rendina. Whatever the source, characteristic radiation from the source is passed through a medium in which free atoms are available. These free atoms are obtained by dissociation of molecules of a sample of the substance under test.

Various means have been employed to dissociate the sample. These methods have included conventional chemical combustion flames. These, unfortunately, are limited in the heat or energy that they are able to impart to the sample and hence the sensitivity of such a system is rather severely limited. Some substances require relatively high dissociation energies. More recently plasmas have been employed to excite the sample under test for emission spectroscopy. One such plasma is described in U.S. Patent 3,242,798 issued Mar. 29, 1966 to Yamamoto. Even plasma sources are not perfect. In their present state of development, they often introduce errors into and decrease the sensitivity of the system, particularly when used in atomic absorption spectroscopy.

One source of errors and lack of sensitivity in atomic absorption measurements is caused by the very physical basic on which atomic absorption spectroscopy depends. The atomic vapor of the sample can emit radiation of the same wave length as is absorbed by the sample. This unusual situation prevails since the electrons of an atom can exist in a number of quantized energy levels or states. The lowest of these energy levels is the so-called ground or unexcited state. If too much energy is imparted to the sample by the energy source, the atoms no longer remain at the ground state, but their electrons are excited to some higher energy level. When the atoms electron(s) return(s) to the ground state from the excited state, radiation is emitted. This radiation may be of the same wave length as that source, causing noise and error in the measurement. Even worse, any atoms while excited are unavailable to absorb the resonant radiation from the source, hence the sensitivity of the measurement is impaired.

In a typical system (combustion flame, plasma jet or other) for the production of an atomic vapor for atomic absorption applications, the sample is introduced into the conversion zone as a spray, mist, or fog, or as solid small particles. In the case of the mist or fog, energy is transferred to or absorbed by the droplets, drying off the more volatile components and leaving a solid particle. Absorption of additional energy by the particle results in the particle reaching the boiling or sublimation point and it becomes independent molecules (a gas). Further absorption of energy by the molecules can produce chemical bond splitting, resulting in free atoms in the ground or excited state, which are capable of absorbing electromagnetic radiation at a resonant frequency. If too much energy has been transferred to the particles or molecules, excitation or ionization may occur, producing an atom which cannot absorb resonant radiation.

The ideal condition for atomic absorption spectroscopy is that suflicient energy be imparted to the molecule to permit dissociation, but not so much so as to excite the sample atoms. This leaves a maximum number of ground state atoms available to absorb the resonant radiation from the source.

Recently, plasmas have found great favor in emission spectroscopy to excite the sample. Unfortunately, however, as was noted many difficulties are encountered in the use of plasmas for atomic absorption spectroscopy either because of too little or too great energy transfer to the sample. As noted, it is desirable in atomic absorption spectroscopy to utilize a system capable of transferring enough energy to the sample (the third body) to dry the droplet, vaporize the solid matter, and split the chemical bonds, but not so much as to excite or ionize the sample. The complete mechanism of energy transfer in an RF plasma sample dissociation system is not fully understood. One of the popular theories holds the plasma itself is a region of highly ionized gas with a considerable density of very energetic electrons. These ions and electrons are accelerated in the intense electric fields produced by the RF supply and thereby possess considerable kinetic energy. If they collide with another species (third body inelastic collision) they may impart all or part of their kinetic energy to the third body which may absorb this energy in several ways, such as increased kinetic energy, ionization, excitation, chemical bond splitting, etc.

The amount of energy available per collision and the number of collisions per unit time define the power transferred from the plasma. The factors affecting the available energy transfer per collision are a function primarily of the gas characteristics. When a monatomic gas is considered, these factors include gas pressure, gas ionization potential and mass of the gas atoms. If a polyatomic gas is used to form the plasma, an additional form of energythe dissociation energy of the polyatomic gas molecules-is available to the third body during each collision. The dissociation energy of a polyatomic gas is present in the free atoms in the form of additional kinetic energy. Upon collision with a third body (the sample) the free atoms transfer this dissociation energy to the third body and then recombine to their original polyatomic molecule. The dissociation energy of these gases is usually quite high compared to the kinetic energy normally available from monatomic gases or the free electrons. Hence a. polyatomic gas, is frequently referred to as a hot gas and a monatomic gas is referred to as a cool gas. For this reason the polyatomic gases are generally used as the plasma gas. Monatomic gases sometimes are used to excite those more easily excited and dissociated substances. Unfortunately, there are not enough different gases available having dilferent dissociation energies to permit the matching of the plasma gas to the element under test. This problem is particularly apparent when testing the more easily excitable elements such as the alkali metals. Other metals of intermediate excitability are sometimes diflicult to detect.

In discussing the ability of plasma to impart energy into the sample, the temperature of the plasma is usually mentioned. Actually this phase has no real meaning. Since the plasma has no equilibrium temperature, it cannot be accurately or meaningfully defined in terms of the conventional temperature units available. The temperature of the plasma often is arrived at through theoretical calculations or one can define the effective plasma temperature by observing the effect of the plasma on a sample introduced into it and correlating or extrapolating these results to those derived from the effect of a thermally heated gas in contact with a sample.

In any event, it is highly desirable to provide a means of controlling the effective temperature of a plasma thereby to improve the sensitivity and accuracy of atomic absorption spectroscopy. It is, therefore, an object of this invention to control the ability of a plasma to impart energy to a sample.

Another object of this invention is to provide a method of controlling a plasmas effective temperature.

An additional object of this invention is to provide an apparatus for controlling the temperature of a plasma for atomic absorption spectroscopy.

SUMMARY OF THE INVENTION A preferred embodiment of this invention provides a method and apparatus for controlling the energy imparted to a sample by a plasma for use in spectroscopic analysis. The method envisions the use of two different gases from which the plasma is formed. These two gases are mixed together and introduced into a plasma generator. The sample itself then is introduced into the plasma. By adjusting the ratio of the first gas to the second gas, the energy transferred from the plasma to the sample may be varied to impart the precise amount of energy to the sample required to dissociate the sample and yet leave the resulting free atoms at the ground state energy level. This provides the maximum number of available atoms which can absorb the characteristic radiation and reduces the amount of characteristic radiation emitted by the sample itself, thereby reducing system noise. I

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation as Well as additional objects and advantages thereof will best be understood from the following description when read in connection with the accompanying drawings, in which the sole figure is a part-schematic and part block diagram of an apparatus that may be employed in performing the method of this invention.

DESCRIPTION OF THE PREFERRED METHOD AND EMBODIMENT The preferred method of this invention may be performed using the apparatus seen in the sole figure in which there is provided a suitable radio frequency (R.F.) generator 10 capable of producing several hundred watts of RF. energy typically in the range of 10 to 1,000 megacycles. The output of the RF. generator 10 is coupled to a tap 12 on an auto transformer 14 for the purpose of raising the RF. voltage. One end 16 of the autotransformer 14, which is closest to the tap 12 in an electrical sense, is grounded. The remaining end 18 of the autotransformer 14, at which the high voltage is available, is coupled to, in this case, a ring electrode 20. A capacitor 22 is placed in parallel with the autotransformer 14 to provide a tank circuit 24 tuned to the frequency of the RF. generator 10. The ring electrode 20 is placed over a quartz tube 28 or other shielding device for containing the plasma generator.

The plasma generator includes a flame tip 30, disposed coaxially within the tube 28 and may be formed of aluminum, silver, or other metal having a high thermal conductivity. The flame tip 30 may be water cooled if desired. The particular design of the plasma generator is not of importance to this invention. A typical design for a plasma generator that can be used is described, for example, in an article entitled Radio Frequency Plasma Emission Spectrophotometry by C. David West and David N. Hume appearing on page 412 of Analytical Chemistry, volume 36, No. 2, dated February 1964. In the alternative, the upper terminal 18 of the autotransformer 14 could be connected to the flame tip 30 and the ring electrode 20 grounded.

In any event, the chamber within the tube 28 surrounding the flame tip 30 is supplied with the gas mixture from which the plasma is to be formed along with the sample which is to be dissociated into ground state atoms by the energy of the plasma. The plasma generator is supplied simultaneously from a first gas supply denoted by the block 32 and a second gas supply denoted by the block 34. The respective first and second gas supplies 32 and 34 are connected through suitable conduits and adjusting valves 36 and 38, respectively to a T junction 40, thence through a nebulizer system denoted by the block 42 to the lower portion of the shielding tube 28. The nebulizer system may be any suitable mechanical or ultrasonic means of atomizing a fluid sample supplied to the fluid inlet 44, denoted sample inlet in the drawing. The atomized or nebulized sample is entrained in the gas mixture flowing to the plasma flame generator through the conduit 46.

A suitable atomizer or nebulizer system for this purpose is described in a letter entitled Direct Continuous Quantitative Ultrasonic Nebulizer for Flame Photometry and Flame Absorption spectrophotometry by Wolfgang I. Kirstan and Groto B. Bertillson which was published in Analytical Chemistry in 1966. The particular system used by Kirstan and Bertillson employs an ultrasonic transducer to which the solution is fed for atomization and subsequent entrainment in the carrier gas from the first and second gas supplies 32 and 34. It is to be noted that it is not necessary that the sample be introduced into the gases. The sample may be introduced directly into the plasma generator if desired. The particular plasma generator used does not form a part of this invention. For the sake of completeness of illustration, characteristic radiation denoted by the dashed line 45 is shown as passing through the shielding tube 28 to the atomic absorption spectrometer 48 for detection and measurement. It is understood that the complete atomic absorption instrument 48 includes a characteristic light source and the atomic vapor system as well as the detecting measuring components.

The method of this invention comprises selecting a gas for the first gas supply 32 which produces a relatively low effective plasma temperature. Typical gases of this type are monatomic gases such as helium, argon, and neon. The gas selected for the second gas supply 34 should be one which produces a relatively high effective plasma temperature or at least a gas producing a higher effective plasma temperature than that of the first gas. Gases of this latter type typically are the diatomic or polyatomic gases such as nitrogen, carbon dioxide, carbon monoxide, hydrogen, and oxygen which provide a relatively hot efiective plasma temperature compared to the first gas. Now by adjusting the flow control valves 36 and 38, the volume ratio of the first gas to the second gas supplied to the T 40 may be varied in accordance with the sample under analysis. The ratio is varied until optimum sensitivity of the atomic absorption spectrophotometer 48 is obtained.

Optimum sensitivity is obtained when (1) the maximum number of free sample atoms are obtained by dissociation of the sample molecules and (2) the sample atoms are at the ground state rather than an excited state. Too high an energy transfer into the sample causes the sample to undergo an undesired amount of excitation. This reduces the sensitivity of the system. Too little energy transfer does not permit the breaking of the chemical bonds of the compounds containing the atoms under test. Too few atoms capable of absorbing the characteristic radiation are produced.

Surprisingly, it has been found that a mixture of nitrogen and argon gases, for example, can provide an effective plasma temperature that varies from that of an argon plasmas elfective temperature alone to that of a nitrogen plasmas eifective temperature alone, depending on the particular gas ratio employed.

Example I In the determination of calcium, whose atoms become excited at a relatively low energy level, a parts per million (ppm) sample of calcium chloride (CaCl was introduced into the plasma generator operating on pure nitrogen in accordance with the prior art teaching. No absorption could be detected using conventional atomic absorption techniques. On the other hand, considerable emission was evident from the atomic vapor. Next pure argon, a known low temperature plasma, was tried. Likewise, no atomic absorption due to the resulting vapor was detected. Next the nitrogen and argon gases were mixed and it was found that a ratio of about 1 part nitrogen to 6 parts argon by volume provided an atomic vapor having excellent atomic absorption characteristics. It was found further that this gas ratio for calcium determination was quite critical. Deviation on either side of this particular ratio reduced the sensitivity and accuracy of the atomic absorption measurement.

Example II A similar situation is observed in the determination of magnesium in a sample of magnesium nitrate (Mg (NO In this case, an optimum nitrogen to argon ratio was found to be approximately 6 parts argon to 2.5 parts nitrogen by volume. From this it can be stated that a hotter, more effective energy transfer plasma than that provided by argon alone is required for the proper measurement or detection of magnesium by atomic absorption techniques.

6 Example IH In still another case sodium from a solution of sodium bicarbonate (NaHCO was determined. Optimum absorption sensitivity was attained using a gas mixture of 1.3 parts argon to 2.3 parts nitrogen by volume.

It is to be noted that as the boiling point of each of these example compounds increases so does the ratio of nitrogen to argon, i.e., the boiling point of CaCl :6H O is 200 C., that of Mg(NO is 330 C., and that of NAHCO is 1390, C. This is to be expected, since as the nebulized vapor of each sample enters the plasma, the water evaporates first leaving solid particles of the sample compound which then vaporize as their respective boiling points are reached.

In each case the proper mixture of gases has permitted the formation of a plasma having the proper effective temperature to supply sufiicient energy to dissociate the sample molecules into free ground state atoms. Although the precise mechanism providing the basis for this invention is not entirely understood, it is believed that the monatomic gases absorb a portion of the energy available in the diatomic gas plasma such that the average effective temperature of the plasma is lowered. The particular gas ratio will vary not only with the element under determination but also with the bonding energy and boiling point of compound in which the element is contained. The optimum gas ratio in each case is determined empirically.

Within limits, a ratio may be established for a particular element and used for subsequent determinations of that element. This is generally true regardless of the compound. The determinations can be made without excessive loss of sensitivity due to lack of sufiicient absorbing atoms and without excessive noise due to a large number of excited atoms. It is preferred however to establish a particular ratio of gases for each and every compound.

Although this invention has been described primarily in terms of mixing a monatomic gas with a polyatomic gas, it is to be noted that any two gases which produce plasmas having different effective temperatures may be used. For example, a diatomic and polyatomic gas may be mixed, two of the same types of gases may be mixed, i.e., two monatomic gases, a monatomic gas and a polyatomic gas may be mixed.

There has thus been described a unique method and apparatus for varying the energy imparted to a sample by a plasma by varying the ratio of plasma gases having different energy transfer capabilities.

While the invention has been disclosed herein in connection with certain embodiments and certain structural details, it is clear that changes, modifications or equivalents can be used by those skilled in the art; accordingly, such changes within the principles of this invention are intended to be included within the scope of the claims below.

What is claimed is:

1. An atomic spectroscopy system for analyzing the atomic composition of a sample comprising:

an atomic absorption spectrometer;

a source of radiation characteristic to a selected element to be detected and measured in said sample, said radiation being directed along a path of travel to said atomic absorption spectrometer;

means interposed in the path of travel of said characteristic radiation for generating a plasma;

means for nebulizing said sample and for introducing said nebulized sample into said plasma generating means;

means for supplying first and second gases to said plasma generating means said first gas being operable to produce a plasma having a predetermined low energy level insuflicient to dissociate said sample, and said second gas being operable to produce a plasma hav- 7 8 ing a predetermined high energy level sufiicient to 3. An apparatus according to claim 1 wherein said sec" dissociate said sample and excite atoms. of said seond gas is a polyatomic gas and said first gas is a diatomic lected element to energy levels above the ground state; gas. I v and i p v References Cited means for adjusting the ratio of said first: and second 5 v UNITED STATES PATENTS gases supphed to said plasma generating means to precisely control the energy transferred to said neb- 3,324,334 6/1967 Raid ulized sample to dissociate said sample and maximize I r n a the number of ground state atomsof a selected .ele- RAYMOND HOSSFELD Pnmary Exammer ment of said sample. v 10 I 2. An apparatus according to claim 1 wherein said sec- 1' I 0nd gas is a polyatomic gas and said first gas is a mon- 313--23'1; 315111;356--96 atomic gas. Y i 

